U.S. Pat. No. 3,389,602 discloses a spacecraft fuel gauging system using capacitive, sound, or radiation probes in conjunction with capillary baffles within a fuel tank in a zero gravity environment to measure the fuel remaining in the tank. In all cases, the probes are in contact with the fuel, and sense wires must be brought out through the fuel tank pressure wall to connect with external instrumentation.
In the present invention, on the other hand, no sensor is in contact with the fuel. Since the sensor 4 is external to the fuel tank 3, no penetration of the pressure vessel 3 is required. Capillary baffles may or may not be present within the tank 3. Adding the present invention to an existing system does not require any changes in the fuel tank 3 or spacecraft propulsion system which uses the fuel 2 contained therein.
U.S. Pat. No. 4,531,405 discloses a method for measuring the level of radioactive fuel within a container, wherein a tube is inserted into the container. The fuel is made to oscillate by applying pressure via the tube. The frequency of oscillation is measured by a pressure detector and an accompanying computer, which converts this information to volume of fuel in the tank. The tube is required as part of the sensing system.
In the present invention, on the other hand, there is no tube or any other device in contact with the liquid fuel 2 within the fuel tank 3. The fuel 2 sloshes when a spacecraft thruster 11 is fired. Accelerations caused by the sloshing fuel 2 are detected by an accelerometer 4 and converted to frequency of oscillations.
U.S. Pat. No. 4,591,117 discloses a system to counteract the nutation of a spin stabilized spacecraft. Said nutation can be caused in part by fuel slosh. An accelerometer is used to measure the nutation on the satellite. However, the accelerometer is not used to measure frequency of oscillation of the fuel within the tank as in the present invention.
In other prior art techniques, the fuel remaining in a spacecraft fuel tank is calculated by counting thruster firings, and by using ground-derived data estimating the fuel that is used for each firing.